PR41991: Accept attributes on defaulted and deleted friends.

Attributes are permitted on friend definitions, but we only checked for
a proper function body, not for the =default / =delete cases.

(cherry picked from commit 5ae6554)

clang/lib/Parse/ParseDeclCXX.cpp

@@ -2716,7 +2716,7 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
     // C++11 [dcl.attr.grammar] p4: If an attribute-specifier-seq appertains
     // to a friend declaration, that declaration shall be a definition.
     if (DeclaratorInfo.isFunctionDeclarator() &&
-        DefinitionKind != FDK_Definition && DS.isFriendSpecified()) {
+        DefinitionKind == FDK_Declaration && DS.isFriendSpecified()) {
       // Diagnose attributes that appear before decl specifier:
       // [[]] friend int foo();
       ProhibitAttributes(FnAttrs);

clang/test/Parser/cxx-default-delete.cpp

@@ -21,3 +21,7 @@ void baz() = delete;
 struct quux {
   int quux() = default; // expected-error{{constructor cannot have a return type}}
 };
+
+struct attrs {
+  [[noreturn]] friend void deleted_with_attrs() = delete;
+};

clang/test/Parser/cxx2a-spaceship.cpp

@@ -16,3 +16,9 @@ void f(X<0> x0, X<1> x1) {
   X<3> e = x0 < x0 <=> x0 << x0;
   X<3> f = x0 << x0 <=> x0 < x0; // expected-warning {{overloaded operator << has higher precedence than comparison operator}} expected-note 2{{}}
 }
+
+struct PR41991 {
+  [[nodiscard]] friend bool operator==(const PR41991&, const PR41991&) = default;
+  [[nodiscard]] friend bool operator!=(const PR41991&, const PR41991&) = delete;
+  [[nodiscard]] friend bool operator<(const PR41991&, const PR41991&); // expected-error {{an attribute list cannot appear here}}
+};